RTE Roundabout Analysis - Technical Data

RTE Roundabout Analysis - Technical Data

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  Roundabouts & TrafficEngineering 20 Crimson Vista Lane Sedona, Arizona 86351 www.roundabouts.us November 19, 2009 David McClure / Jerry Joseph Dinzes Kings Beach Business & Citizens Alliance PO Box 349 Tahoe Vista, California 96143 P: (530) 546-8297 RE: Kings Beach SR28/Coon Rbt Capacity Review Dear Mr. McClure / Dinzes: As you requested, Roundabouts & Traffic Engineering (RTE) has conducted a cursory review of the existing capacity requirements for a proposed modern roundabout at the intersection of SR 28 / Coon Street in Kings Beach, California. The purpose of this brief technical letter is to determine the capacity requirements of the intersection under the existing 2008 traffic volumes as provided in the EIR. In particular, these capacity analyses can be used to determine the effectiveness of the current proposed conceptual roundabout layout with respect to capacity. A full roundabout design review (Peer Review) by a recognized roundabout design specialist is typically required by most jurisdictions nationwide and should be completed for this project prior to final PS&E document generation if the jurisdictions are to proceed with any civil engineering of a roundabout alternative for the corridor. DATA & CONDITIONS As stated in RTE’s scope of work: “Due to a limited budget, RTE’s scope of work will consist of a cursory peer review of one of the roundabouts in the project corridor primarily consisting of the capacity analyses and lane configuration requirements of the roundabout based on future pre-approved design year volumes.” However, the KBBCA elected to use the EIR’s representative design year 2008 PM peak hour traffic volumes since future 2028 design year volumes already demonstrate failing capacity conditions as a single lane roundabout. Hence, per your request, RTE has summarized the results of the 2008 PM peak hour capacity analyses herein for your use. Roundabouts & Traffic Engineering ARIZONA OFFICE 20 Crimson Vista Lane Sedona, Arizona 86351 Direct: (928) 284-0295 Main: (928) 284-0366 www.roundabouts.us
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  SR 28/Coon CapacityAnalyses Page 2 November 19, 2009 Roundabout Review Services Roundabouts & Traffic Engineering 20 Crimson Vista Lane Sedona, Arizona 83651 www.roundabouts.us As directed, RTE utilized the vehicular turning movement volumes provided in the Kings Beach Urban Improvement Project Traffic Report (LSC, dated 1/25/2007) under Table 17: Peak Hour Summer 2008 Intersection Turning Movement Design Volumes (LSC, page 27) for the SR 28 / Coon Street intersection. These volumes developed by years of local data, traffic studies, and observations by LSC staff, are based on 2002 traffic counts (30th highest peak hour day of the year) and have numerous adjustments and assumptions therein including applied growth rates, conversion of two-way streets to one-way, and shifted traffic assumptions. It is not within RTE’s scope to dispute the use of these adjustments and assumptions therein, whereas RTE assumes all adjustments and assumptions are approved, appropriate, and accurate for the capacity analyses conducted herein. RTE is familiar with the SR 28 Corridor, site conditions, and constraints since RTE was hired in 2006 by Dokken Engineering to develop preliminary feasibility sketches of two multi-lane roundabouts with small diameters along the SR 28 Corridor. Subsequently thereafter 2006, decisions were made to analyze only single lane modern roundabouts since the multi-lane roundabouts were considered by the jurisdiction infeasible due to a loss of parking spaces and cost of adjacent property acquisitions. 2008 CAPACITY REVIEW & ANALYSES After obtaining and reviewing all of the pertinent information regarding the project and provided PM peak hour traffic volumes, a geometric analysis of the proposed roundabout at the intersection of SR 28 / Coon Street using the roundabout design software tool called RODEL (Roundabout Delay) was conducted. In most roundabout knowledgeable states nationwide, the RODEL software is the required analysis software and is considered superior to other software (such as SIDRA or Vissim) since it is based on an empirical analysis method and yields more accurate and consistent results over the entire spectrum of potential volumes and roundabout geometry. Caltrans has allowed both RODEL and SIDRA for the capacity analyses of modern roundabouts. The RODEL calculations verify the initial recommended number of entry lanes and capacity requirements for the roundabout approaches based on the design year traffic volumes and the geometric design parameters of the intersection. In general, RODEL calculates the required geometry for the roundabout to function within the desired capacity or, as in this case, to determine if the proposed roundabout design or geometry of the roundabout will be adequate with respect to capacity and delay. Although only one set of volumes (2008 Summer PM Peak Hour) were requested as part of this capacity review, separate RODEL calculations were completed for the intersection location to review how the roundabout will operate under both 2008 peak hour traffic
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  SR 28/Coon CapacityAnalyses Page 3 November 19, 2009 Roundabout Review Services Roundabouts & Traffic Engineering 20 Crimson Vista Lane Sedona, Arizona 83651 www.roundabouts.us conditions and slightly higher volumes using simple growth factors since this review occurs after a 2008 design year implementation. In addition, separate RODEL calculations were also performed at an 85th percentile confidence level in order to: (1) check the design to verify if the initial design parameters and conceptual layout would be adequate under the anticipated volumes and (2) account for additional urban congestion such as pedestrians, parking, close driveway spacing, commercial structures close to the roadway, driver distractions of a narrow commercial corridor, resort town distractions, and other additional side friction or capacity reducing factors. In addition, the traffic volumes did not indicate a typical bell curved peak hour, rather several hours of consistent peak hour volumes. Therefore, 100% of the peak hour volumes were assumed to already be present on the roadway before and after the peak hour analyses. The ability to adjust the software’s analysis parameters (such as a higher percentile confidence level) is extremely useful and available within RODEL wherein other software only use the standard 50th percentile confidence level inherent in all capacity models. Typically, the 85th percentile confidence level is used and considered simply as a “design check” to ensure adequate capacity of the roundabout is available through both the lane configuration and the geometric values used in the design. However, under more urban conditions, this higher confidence level accounts for additional side friction factors. Some jurisdictions such as NYDOT require analyses to be only presented at the 85th percentile in more congested or urban areas. Hence, the capacity analyses conducted for this intersection (SR 28 / Coon Street) were completed at both a 50th and 85th percentile confidence levels. Since roundabout designs have already been developed, RTE calculated and measured the geometric design parameters for the input into the RODEL software with respect to the following elements: ! Entry width (E) – limited to a single lane entry value ! Average Effective Flare Length (L’) – as measured ! Half Width of the Roadway (V) – limited to a 12ft (3.66m) lane ! Entry radius (R) – as measured ! Entry angle (phi) – as measured ! Roundabout Diameter / ICD (D) – as measured
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  SR 28/Coon CapacityAnalyses Page 4 November 19, 2009 Roundabout Review Services Roundabouts & Traffic Engineering 20 Crimson Vista Lane Sedona, Arizona 83651 www.roundabouts.us The geometric design parameters used in these analyses were measured from the most current design provided to RTE (see attached). Truck percentages were based on the provided Caltrans truck data at mile posts 9.34 and 11 on SR 28. Based on the established design criteria for the roundabout intersection analyses, the RODEL software program and roundabout engineering judgment, RTE determined the following capacity results for the single lane roundabout intersection at SR 28/Coon Street: 1. Based on the 2008 design year volumes and a standard 50th percentile confidence level, the single lane roundabout would function at an overall LOS “C” with 23.6 seconds of average delay occurring during the summer PM peak hour. The associated delays mostly occur in the EB and WB directions on SR 28 with queue lengths forming 250 to 375 feet eastbound and 150 to 175 feet westbound. Please refer to the attached Figure 1 for this scenario’s detailed results. 2. Based on the 2008 design year volumes and an 85th percentile confidence level, the single lane roundabout would function at an overall LOS “F” with 158.7 seconds of average delay occurring during the summer PM peak hour. The associated delays mostly occur in the EB and WB directions (both operating at LOS F) with queue lengths forming 2,200 to 3,225 feet (roughly ½ mile) eastbound and 1,125 to 1,475 feet (roughly ¼ mile) westbound. Please refer to the attached Figure 2 for this scenario’s detailed results. The results of these analyses indicate a single lane roundabout would not be adequate for capacity purposes under 2008 conditions. Considering this “Urban Improvement Project” and urban conditions of the Kings Beach Corridor, the 85th percentile calculations are appropriate for the capacity analyses of this site. Unfortunately, SIDRA underestimates the capacity needs of a roundabout at relatively lower volumes (such as in this case) and does not have the abilities to compensate for additional side friction or urban factors (such as pedestrians, parking, close driveway spacing, commercial structures close to the roadway, driver distractions of a narrow commercial corridor, resort town distractions, and other additional side friction or capacity reducing factors). Even with the 85th percentile calculations assumed as only a design check, the results clearly indicate the single lane roundabout is under-designed with respect to the geometric design and required lane configuration. Taking into account the close proximity of another roundabout roughly 600 feet away, the capacity of each roundabout will be affected by the other’s limitations in capacity. In addition, reviewing the resulting queue lengths of this roundabout, it is clear the queues will “back up” or stack into the other roundabout at only 600 feet away. As each roundabout begins to queue, the capacity issue / degradation is compounded at the upstream roundabout and between roundabouts.
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  SR 28/Coon CapacityAnalyses Page 5 November 19, 2009 Roundabout Review Services Roundabouts & Traffic Engineering 20 Crimson Vista Lane Sedona, Arizona 83651 www.roundabouts.us Based only on the capacity analyses, two lane approaches are recommended for the eastbound and westbound directions and single lane approaches northbound and southbound are needed at this roundabout. However, a partial multi-lane roundabout at this location has been evaluated and dropped from the viable alternatives list (as noted above) due to the lack of right-of-way to the south or impacts to adjacent properties in the southeast and southwest quadrants (as specified in the EIR). Ultimately, the final lane configuration of the intersection is determined at the discretion of the governing jurisdictions. This assumes all alternatives and constraints have been adequately analyzed and reported. 2010+ CAPACITY REVIEW & ANALYSES The intersection was also analyzed under slightly higher volumes using simple growth factors since this review occurs in late 2009 and the 2008 design year assumes the project to be completed / built. Therefore, two additional scenarios are shown below first with a simple 5% increase in the traffic volumes and second increasing the volumes until the 50th percentile confidence level demonstrates failure, which occurred at a just over a 10% increase in 2008 design year volumes (85% percentile confidence level already demonstrates failure). 3. Increasing the 2008 design year volumes by only 5% and using a standard 50th percentile confidence level, the single lane roundabout would function at an overall LOS “D” with 36.9 seconds of average delay occurring during the summer PM peak hour. The associated delays mostly occur in the EB and WB directions on SR 28 with queue lengths forming 500 to 675 feet eastbound and 225 to 275 feet westbound. Please refer to the attached Figure 3 for this scenario’s detailed results. 4. Increasing the 2008 design year volumes by only 5% and using an 85th percentile confidence level, the single lane roundabout would function at an overall LOS “F” with 238.5 seconds of average delay occurring during the summer PM peak hour. The associated delays mostly occur in the EB and WB directions (both operating at LOS F with 5 to 11 minutes of delay EB and 3 to 7 minutes of delay WB) with queue lengths forming 3,350 to 4,925 feet (nearly 1 mile) eastbound and 1,925 to 2,750 feet (roughly ½ mile) westbound. Please refer to the attached Figure 4 for this scenario’s detailed results. 5. Increasing the 2008 design year volumes by 13% and using a 50th percentile confidence level, the single lane roundabout would function at an overall LOS “F” with 88.4 seconds of average delay occurring during the summer PM peak hour. The associated delays occur in the EB and WB directions with queue
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  SR 28/Coon CapacityAnalyses Page 6 November 19, 2009 Roundabout Review Services Roundabouts & Traffic Engineering 20 Crimson Vista Lane Sedona, Arizona 83651 www.roundabouts.us lengths forming 1,450 to 2,050 feet eastbound and 575 to 750 feet westbound. Please refer to the attached Figure 5 for this scenario’s detailed results. 6. Increasing the 2008 design year volumes by 13% and using an 85th percentile confidence level, the single lane roundabout would function at an overall LOS “F” with 373.8 seconds of average delay occurring during the summer PM peak hour. The associated delays occur in the EB and WB directions (8 to 18 minutes of delay EB and 6 to 12 minutes of delay WB) with queue lengths forming 5,350 to 7,900 feet (1 to 1.5 miles) eastbound and 3,525 to 5,225 feet (roughly 2/3 to 1 mile) westbound. Please refer to the attached Figure 6 for this scenario’s detailed results. As indicated above and in the attached RODEL output (Figures 3-6), the results of these analyses indicate a single lane roundabout would not be adequate for capacity purposes under more current 2010 traffic conditions (5% growth factor) with failing level of service during the 30th highest peak day assumptions. Furthermore, with slightly more than a 10% increase in traffic volumes, the intersections fall further into failure and create significantly long delays and lengths of stacked vehicles along SR 28 reaching over a mile in length. RTE fully recognizes the obvious site constraints at this intersection and is not involved with this projects decision process, local politics, right-of-way acquisitions, or rational for reducing the capacity of the existing SR 28 corridor. It should be noted herein, this is an objective 3rd party review of the capacity requirements and operations of the intersection. If the roundabout was redesigned to allow more appropriate entry speeds of 25 miles per hour (opposed to 19 to 21 mph entry speeds), larger entry radii and smaller entry angles, the capacity of the roundabout could be improved and would extend the life of the single lane roundabout at least a few years. In addition to tight roundabout geometry creating capacity issues, RTE does not recommend truck aprons in the entry / approach of a modern roundabout wherein safety concerns apply. A larger roundabout diameter is recommended for larger trucks such as in this case with WB-67s or Caltrans design vehicles (FHWA Roundabout Guide recommends a 115 foot minimum diameter). In addition, the proposed roundabout design has extremely small entry radii at 17 and 18 meters (56 to 59 feet) which are not good values or recommended for a single lane roundabout. Most roundabout guides do not recommend a single lane entry to be less than around 70 feet. If possible, redesign of the conceptual layout is recommended to better accommodate trucks and speeds. However, it should be noted this cursory review does not perform a full roundabout design review with complete recommendations, rather, only calculates the existing capacity of the potential roundabout design based on the provided volumes. The
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  SR 28/Coon CapacityAnalyses Page 7 November 19, 2009 Roundabout Review Services Roundabouts & Traffic Engineering 20 Crimson Vista Lane Sedona, Arizona 83651 www.roundabouts.us provided roundabout design seems to be in preliminary form (not completed) and has not been verified by RTE to function adequately with respect to operational safety, proper speeds, proper truck capabilities, and other primary roundabout geometric parameters. If this project proceeds further or if at any point during this project further design or peer review assistance is needed, RTE encourages the jurisdictions to allow RTE to either produce the roundabout designs and non-geometric design details with the project team or perform detailed peer reviews of the roundabouts to ensure these roundabouts will operate safely with more capacity and be implemented adequately in the field. RTE hope this brief letter has provided you with all of the essential preliminary capacity information that was requested for this intersection. If you, the Kings Beach Business & Citizens Alliance, and/or the jurisdictions involved with this project (Placer County, TRPA, and Caltrans) have any questions or need anything further regarding this project or roundabouts in general, please feel free to contact me. Respectfully Submitted, Roundabouts & Traffic Engineering, Attachements: Capacity Analyses - RODEL Output Figure 1: 2008 Summer Peak – 50th PCL Figure 2: 2008 Summer Peak – 85th PCL Figure 3: 2008 + 5% Grown Factor – 50th PCL Figure 4: 2008 + 5% Grown Factor – 85th PCL Figure 5: 2008 + 13% Grown Factor – 50th PCL Figure 6: 2008 + 13% Grown Factor – 85th PCL Preliminary Design Layout Used For Analyses
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  SR 28/Coon CapacityAnalyses Page 8 November 19, 2009 Roundabout Review Services Roundabouts & Traffic Engineering 20 Crimson Vista Lane Sedona, Arizona 83651 www.roundabouts.us Figure 1: 2008 Summer Peak (30th Highest) – 50th Percentile Confidence Level Figure 2: 2008 Summer Peak (30th Highest) – 85th Percentile Confidence Level
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  SR 28/Coon CapacityAnalyses Page 9 November 19, 2009 Roundabout Review Services Roundabouts & Traffic Engineering 20 Crimson Vista Lane Sedona, Arizona 83651 www.roundabouts.us Figure 3: 2008 Summer Peak + 5% Growth Factor – 50th Percentile Confidence Level Figure 4: 2008 Summer Peak + 5% Growth Factor – 85th Percentile Confidence Level
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  SR 28/Coon CapacityAnalyses Page 10 November 19, 2009 Roundabout Review Services Roundabouts & Traffic Engineering 20 Crimson Vista Lane Sedona, Arizona 83651 www.roundabouts.us Figure 5: 2008 Summer Peak + 13% Growth Factor – 50th Percentile Confidence Level Figure 6: 2008 Summer Peak + 13% Growth Factor – 85th Percentile Confidence Level